Method of forming U.V. stabilized porous pipe

ABSTRACT

Porous pipe formed of particles of vulcanized rubber dispersed in a binder resin such as polyethylene are stabilized to reduce degradation by ultraviolet light by dispersing in the binder resin an effective amount of an ultraviolet stabilizer such as 0.1 to 10% by weight of carbon black.

This is a Division of application Ser. No. 07/930,345, filed Aug. 14,1992 now U.S. Pat. No. 5,299,885, Apr. 5, 1994.

TECHNICAL FIELD

The present invention relates to porous irrigation hose and, moreparticularly, this invention relates to porous irrigation hose havingstability to ultraviolet radiation.

BACKGROUND OF THE INVENTION

Porous pipe has been developed that weeps or drips water along itslength. The hose wall is a composite formed of a dispersion of a majorportion of prevulcanized elastomer particles dispersed in a continuousbinder phase, usually a thermoplastic resin such as polyethylene.Channels or crevices are formed in the wall due to the lack of excessbinder and the incompatibility between the binder and the dispersedparticles. When the pipe is placed under pressure, the wall expandsslightly and water traverses the wall and forms beads or drops on theoutside surface which drip to form a continuous line source of water.Porous hose can also be used underground as a line source of irrigationat the root level or around foundations to remove excessive moisturefrom the soil. Representative patents describing the manufacture ofporous pipe are U.S. Pat. Nos. 4,517,316, 4,616,055 and 4,615,642.

Porous pipe is finding increasing use as soaker hose manufactured withor without hose fittings for use in retail do-it-yourself (DIY), lawnand garden and professional landscape and agricultural applications.Porous hose used in outdoor applications is found to weather and becomebrittle and crack after extended exposure to the outdoor ambientenvironment. It was believed that the aging was due to ozone in the airand was not caused by incident radiation. It is known that hydrocarbonresins such as polyethylene are degraded by ultraviolet radiation. Theradiation can create free radicals resulting in chain scission or thecreation of peroxy or other radicals that can degrade the polymerchains. However, it is also known that adding 3 to 5% by weight ofcarbon black pigment to transparent resins such as polyethylenestabilizes them from attack by ultraviolet radiation. Porous pipe is acomposite containing at least 60% by weight of reclaimed rubberparticles which themselves contain about 30% carbon black. Therefore,the composite hose already contains about 20% carbon black. The productis opaque. It was believed that this opacity and the high content ofcarbon black would provide all the ultraviolet absorption that the hoserequired.

STATEMENT OF THE INVENTION

It has now been discovered that the addition of ultraviolet absorbers orstabilizers to the continuous binder phase results in substantialreduction in degradation of the physical properties of the wall ofporous hose due to ultraviolet radiation. It was discovered onmicroscopic examination of the product that the binder resin formed athin transparent film encapsulating the rubber particles. Theprevulcanized particles firmly bind the carbon black and do notcontribute any substantial carbon black to the binder phase. The carbonblack is not available for absorption of ultraviolet light. The incidentultraviolet light apparently enters and degrades the transparent binderfilm leading to stress cracking and failure. The stress cracking isthroughout the wall of the pipe. The ultraviolet light must flow throughthe binder film by light pipe principles and generate radicals thatdegrade the polyethylene or other binder polymer. The thinness of thefilm of transparent binder resin may act to concentrate or attenuate theeffects of the incident ultraviolet radiation and accelerate theageing-stress cracking process.

The addition of a small amount of an absorber or stabilizer is found tosignificantly increase the environmental stress crack resistance of theporous pipe. After accelerated exposure for an extended period, porouspipe formulated without a U.V. absorber cracked on bending. Porous pipeformulated to contain a U.V. absorber according to the invention couldbe bent without cracking. Samples of regular and U.V. stabilized porouspipe were tested for 6 months exposure to incident Arizona sunlight. Theregular pipe was brittle and cracked. The U.V. stabilized pipe was stillflexible and uncracked after six months exposure.

The U.V. stabilized pipe of the invention will substantially extend theservice life of porous pipe exposed to ultraviolet light, especially inthe southern and western United States which have many more days ofclear skies and direct sunlight exposure at ground level. The U.V.stabilized pipe of the invention will also be able to be deployed incurved or bent configurations since the stress-crack resistance issubstantially improved.

These and other features and many attendant advantages of the inventionwill become apparent as the invention becomes better understood byreference to the following detailed description when considered inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an end view of a porous pipe produced in accordance with theprior art.

FIG. 2 is an enlarged partial view of the wall of FIG. 1 showing thefilm of binder resin on the rubber particles; and

FIG. 3 is an enlarged view of the wall of a product produced inaccordance with the invention showing the stabilized binder film.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 1 and 2, porous pipe 10 has a wall 12 formed of adispersion of discrete elastomer particles 14 dispersed in a binderresin 16. The binder resin coats the particles 14 with a thintransparent film 18. The binder films 18 are connected to form acontinuous matrix which holds the product together. In the prior artproduct, the binder film 18 was unpigmented and allowed incidentradiation 20 to enter and degrade the film 18. Ultraviolet radiation cancreate free radicals causing chain scission of the hydrocarbon chains ofthe binder resin. Ultraviolet radiation can in the presence of oxygenfrom the air result in the formation of radicals that can form carbonylgroups or cause embrittlement by cross-linking or scission of thebackbone chains of the binder resin.

As shown in FIG. 3, when the binder resin contains an ultravioletradiation stabilizer such as carbon black particles 24, the ultravioletradiation 20 is absorbed or its reaction products are passivated suchthat the chemical and physical properties of the binder resin are notdegraded.

The ultraviolet stabilizer is a material that can be dissolved ordispersed in the binder resin and is not degraded at the temperatureexperienced during extrusion, typically from 250° F. to 450° F. Thestabilizer is generally present in a minor amount from 0.1 to 10% byweight of the binder resin, generally from 0.1 to 3% by weight.

The ultraviolet absorber can be a dispersion of carbon black. The use ofcarbon black as an ultraviolet absorber for polyethylene and vinylresins is well known. Carbon black is known to exist in polycyclic formand to contain hydroxyl and oxygen substitution. Carbon black may infact function as a polyhydric phenol providing both ultraviolet andthermal stabilization.

Other recommended ultraviolet radiation stabilizers are benzophenonesand amines such as benzotriazoles. Representative are2-hydroxy-benzonphenones selected from compounds of the formula:##STR1## Where R may be selected from H, lower alkyl of 1 to 12 carbonatoms or lower alkoxy of 1-12 carbon atoms, preferably in the4-position. 2-hydroxyl-4-n-octoxybenzophenone is a nonmigratory additivethat preferentially absorbs ultraviolet radiation and dissipates it asnon-destructive energy. It effectively inhibits photoinitiated physicalfailure of polyethylene films. Its solubility in solvents andplasticizers permits it to be incorporated in vinyl films or foamedproducts. This absorber is generally utilized in amounts from 0.1 to 1%by weight, preferably from 0.3 to 0.5% by weight. Development of 0.1%carbonyl content in polyethylene films or molded products coincides withnoticeable degradation in physical appearance and properties.Polyethylene rubber composite hose will have less than 0.1% carbonyl inthe polyethylene binder film after 12 months exposure to Arizonasunlight.

Another type of recommended U.V. absorber for polyethylene or vinylresins are 2-Hydroxy-benzotriazoles of the formula: ##STR2## in which R¹is selected from hydrogen, lower alkyl of 1-12 carbon atoms or loweralkoxy of 1-12 carbon atoms, preferably in the 5-position. 2-(2-Hydroxy5-t-octylphenyl)-benzotriazole is an effective ultraviolet lightstabilizer for polyvinyl chloride resins at the 0.1 to 0.5% by weight.

Oligomeric ultraviolet stabilizers of the hindered amine class canprovide both excellent stabilization at low concentration to ultravioletlight and thermal and ultraviolet protection to polyolefin polymersystems under outdoor exposure conditions.

The composition for extruding porous pipe essentially contain from about30% to about 40% by weight of thermoplastic binder resin, about 60-70%by weight, preferably 63-67% by weight of discrete elastomer particlesand an ultraviolet stabilizer uniformly dissolved or dispersed in thebinder resin in an amount effective to stabilize the resin from thedegradation effects of ultraviolet light for at least 6 months. Thecarbonyl content of the binder resin is preferably below 0.1% by weightfor at least 6 months. The composition may contain optional ingredientssuch as small amounts of lubricants or slip agents to aid in mixing andextruding the pipe.

As disclosed in U.S. Pat. Nos. 4,517,316 and 4,616,055, the moisturecontent of the rubber particles is maintained below 1.0% by weight,usually below 0.5% by weight and preferably as low as possible, such asno more than 0.1% by weight. Moisture can be removed from the rubberparticles by passing them through a drier before feeding to the extruderor drying them under ambient conditions such as in a hot, dry ambient asexperienced in the Southwestern United States.

The thermoplastic binder resin has a melting temperature suitable forextrusion of the porous pipe, generally from 250° F. to 450° F., usuallyfrom 275° F. to 350° F. The resins can be polyolefins or polyvinylresins. Polyethylene resins are preferred due to their flexibility,inertness and extrudability. The polyethylene can be of the high densityor low density types. Low density resins having a density below 0.95usually 0.92 to 0.94, provide products with better flexibility andelongation, especially linear low density (LLD) polyethylenes which haveless chain branching and a narrower molecular weight distribution.

Polyethylene is less hygroscopic then the rubber particles and ispresent in one-half of the amount of the rubber. Unless the surface ofpolyethylene particles contains excessive moisture exceeding 0.5% byweight, it may not be necessary to predry the polyethylene beforeextrusion. The polyethylene is usually provided as short rods or pelletshaving a particle size of about 30-50 mesh.

The elastomer particles are prevulcanized. They remain discrete and donot dissolve in the binder resin. The particles are preferably reclaimedrubber recycled from the tread portion of used tires or other sources.They also can be virgin rubber such as flashings from tire manufacture.The rubber is ground to a particle size excluding large particles aboveabout 40 mesh. Excessive fines should be avoided. A preferred product isa 30 mesh granular reclaimed rubber having a moisture content asdelivered of no more than 0.5% by weight.

The mixture of binder resin and rubber particles is formed into porouspipe by continuous extrusion. Venting is not necessary to produce highquality porous hose. The extruder barrel is heated to a temperatureabove the melting point of the binder resin. In the case of low densitypolyethylene the extruder barrel is heated to a temperature from 250° F.to 450° F. The die is generally separately heated to a temperature from300° F. to 400° F. The extruder may contain compression and mixingsections or can contain a single screw with continuous spiral thread.

As the hot, compressed extruded product leaves the die, the rubberparticles expand to their normal volume. The porous hose passes througha long cooling trough containing cooling water generally at or near roomtemperature, usually from 50° F. to 80° F.

The resin and rubber particles can be premixed in a mixer such as aribbon blender. The soluble stabilizers can be added at the mixingstage. It has been found difficult to disperse the carbon black inpolyethylene in the mixer in the presence of the rubber particles. Thecarbon black can be predispersed in the polyethylene.Polyethylene-carbon black (PE-CB) dispersions in pellet form containing30-50% by weight of carbon black are commercially available. These PE-CBpellets are found to readily disperse the carbon black in the largeramount of binder resin. The PE-CB pellets can be added at the mixingstage or all ingredients can be separately fed to the hopper to theextruder. Since the binder is thermoplastic it is also possible toregrind off-spec product and recycle it to the extruder. The regrind isgenerally ground to 30 mesh rubber particles. Since the proportions inthe recycle product are the same as the composition being extruded, theratio of feed of the fresh ingredients need not be adjusted. Generally,the recycle product is limited to no more than 5% by weight of thebatch, generally about 2% by weight.

EXAMPLE 1

Porous pipe having an O.D. of 0.825 inches and a wall thickness of 0.001inch was extruded in an unvented pipe extruder having a barrel heated toa temperature of 350° F. and a die heated to a temperature of 350° F.The water in the chiller bath was at 60° F. The extrusion rate was 150ft/min. The composition of the mixture was 35% of linear low densitypolyethylene and 65% of 30 mesh reclaimed rubber have a moisture contentbelow 0.5% by weight.

EXAMPLE 2

Example 1 was repeated utilizing a feed composition the same as Example1 to which was added 7 additional parts per hundred parts of pelletscomprised of 60/40 LLD polyethylene/carbon black. The polyethylene inthe pellet need not be linear low density polyethylene but can by anyresin that is freely mixable and blendable with LLD-PE. This added 2.8parts of carbon black to 107 parts of the mixture (about 2.6% of themixture) or 2.8 parts of carbon black to 39.2 parts of polyethylene(about 7.1% based on polyethylene).

EXAMPLE 3

A mixture of 65 parts by weight of 30 mesh rubber reclaim, 35 parts byweight of LLD-PE and 3 parts of carbon black powder were premixed in amixer until blended. The mixture was extruded to form porous pipe inwhich the binder phase contains a uniform dispersion of carbon black.

Straight and bent samples of porous pipe produced in accordance withExamples 1 and 2 were placed on the roof of a facility in the Phoenix,Ariz. area for six months. The straight and bent samples of Example 1pipe both showed degradation with the bent sample showing excessivecracking along the inner and outer walls of the bend. The straight andbent samples of Example 2 pipe did not exhibit degradation due toambient exposure to sunlight.

Bent samples of porous pipe produced according to the procedures ofExample 1 and 2 were subjected to accelerated ageing by alternativelyimmersing the samples in 10% Igepal solution at 160° F. according to theEnvironmental Stress Crack Resistance Weatherometer Test (ASTM 2239-81)and then exposing the samples to a bank of hot lights.

The unstabilized sample of Example 1 showed severe stress-crack failurewhile the stabilized sample of Example 2 did not exhibit stress-crackfailure.

It is to be realized that only preferred embodiments of the inventionhave been described and that numerous substitutions, modifications andalterations are permissible without departing from the spirit and scopeof the invention as defined in the following claims.

We claim:
 1. A method of forming porous pipe comprising the stepsof:forming a mixture of a thermoplastic binder resin selected from apolyolefin resin or a vinyl resin containing a dispersion of 60% to 80%by weight of vulcanized elastomer particles and said binder resincontaining 0.1 to 10% by weight of an ultraviolet stabilizer based onthe weight of the pipe in an amount effective to substantially reducedegradation of the binder resin due to ultraviolet radiation; extrudingthe mixture through a heated pipe die; and cooling the extruded pipe ina cooling trough.
 2. A method according to claim 1, in which theultraviolet stabilizer is carbon black and the carbon black ispredispersed in a carrier resin miscible with the binder resin.
 3. Amethod according to claim 2 in which the binder resin and carrier resinare polyethylene.
 4. A method according to claim 1 in which theelastomer particles are present in an amount from 60% to 70% by weight.5. A method according to claim 4 in which the elastomer particles arepresent in an amount from 63% to 67% by weight.
 6. A method according toclaim 5 in which the elastomer particles are uniformly sized.
 7. Amethod according to claim 6 in which the particles pass a 30 meshscreen.
 8. A method according to claim 1 in which the polyolefin is apolyethylene.
 9. A method according to claim 8 in which the polyethyleneis a low density polyethylene.
 10. A method according to claim 9 inwhich the polyethylene is a linear low density polyethylene.
 11. Amethod according to claim 1 in which the ultraviolet stabilizer isselected from carbon black, phenols or amines or mixtures thereof.
 12. Amethod according to claim 11 in which the stabilizer is carbon black.13. A method according to claim 1 in which the moisture content of themixture before extrusion is below 0.5 percent by weight.